A sleeper is a small black furred creature about the size of a humanoid hand, equipped with a lethal stinger on its back and six tentacles used for locomotion that allow it to leap astonishing distances. The creature is used as a means of assassination and is implanted inside an unwitting host, wiping their memory and waiting for the opportunity to emerge and strike at its target. During its period of dormancy, the host is unaware of the sleeper within or that their actions are being subtly influenced. Unlike a keeper, a sleeper can - once it is fully awake - take direct control over the host in order to get into striking distance.

In late 2262, the Drakh implanted Rem Lanas - a Centauri chosen at random - with a sleeper and sent him to live in Babylon 5's Downbelow. The intent was to assassinate ISA PresidentSheridan as he lead a group of Interstellar Alliance officials through the area as part of an initiative to improve conditions on the run down areas of the station. The attempt was thwarted after EmperorLondo Mollari was told of the scheme by the Drakh Shiv'kala, as a test that he had learned his place. Mollari failed the test when he managed to circumvent his keeper and obliquely warn AmbassadorVir Cotto of the plot by mentioning the late emperor Kran (who had met a similar fate over a century ago) and the Lanas's name. Vir managed to intercept Rem as Sheridan's procession drew near and the sleeper was killed with the unexpected intervention of the techno-mageKane.